In recent years it has been shown that in eukaryotic cells, proteins may be modified by the addition of isoprenoid groups to the amino acid backbone. The isoprenoid groups, in particular farnesyl and geranylgeranyl, are attached at the carboxy terminus of the protein, by a thioether linkage to a terminal cysteine residue.
Proteins which are modified in this way have increased affinity for specific membranes and certain integral membrane proteins. The addition of a prenyl group increases hydrophobicity. The hydrophobicity is further increased by cleavage of the three amino acids C-terminal to the cysteine, and by methylation of the now terminal cysteine.
The addition of prenyl groups may be required for activity of the protein. In the ras family of proteins, addition of a farnesyl group is required for normal activity. A mutated form of ras, which can be modified by the addition of geranylgeranyl, has transforming activity but not normal function. Blocking all prenylation results in a soluble protein which is not active.
Specific enzymes have been shown to catalyze the addition of farnesyl and geranylgeranyl groups. A consensus sequence for a protein to be prenylated is the "CAAX" box, where C is cysteine, A is an aliphatic amino acid, and X may be any amino acid. The amino acid "X" determines whether the protein will be modified with farnesyl or geranylgeranyl. A CC or CXC carboxy terminal peptide motif can also signal for the addition of geranylgeranyl. Most prenylated proteins are geranylgeranylated, and a minority are modified with farnesyl groups. Proteins which are modified by farnesylation include ras, lamins and the .gamma. subunit of transducin.
There are at least two proteins which are active in transferring geranylgeranyl groups to proteins. However, there is only one enzyme in mammals which is active in transferring farnesyl to proteins, farnesyl protein transferase (FPTase). The protein, which has recently been cloned, is an .alpha., .beta. heterodimer. The protein substrate binds to the .beta. subunit, while the .alpha. subunit binds farnesyl diphosphate. The biological activity of farnesylated proteins makes it of interest to determine whether farnesyl protein transferase activity can be reduced.